Broken Crankshaft

by John

The lovely 1:4 scale Fowler traction engine which I bought in 2017 has had little use in the past 3 years, so I have decided to sell it. The lack of use was mainly due to Covid shut down of steam meets and exhibitions. While Covid restrictions have ceased, my interests have changed, and I now prefer to concentrate on smaller, stationary steam engines.

But first I needed to renew the boiler certification. The boiler is constructed from 4mm thick copper, silver soldered, and was made by an experienced engineer, so I do not expect any significant problems with the re-certification. Just to be sure, I ran the engine on compressed air. Immediately I noticed that the flywheel was rotating more slowly than the crankshaft. The cause was a sheared pin which joins two segments of the crankshaft.

The flywheel has always had a slight wobble, but now it was more pronounced. Obviously the crankshaft needed to be repaired or replaced. Initially I hoped that all that would be required was a new pin. It was a 1/8″ roll pin, and I hoped that I could tap it out and simply replace it.

I have the original construction plans for the engine, and those plans recommend a solid crankshaft in the interests of longevity. However, the original maker had chosen to make a built up crankshaft, securing the 8 joins with roll pins, and probably Loctite.

I contacted the original maker of the engine, an elderly gentleman living interstate, and we had a long and pleasant conversation. He was surprised that the crankshaft had failed, but did not recall the details of the construction. He strongly recommended removing the crankshaft from the engine and working on it in the workshop, a decision which I had already made.

Long story shortened. It took me 4 hours to remove the crankshaft, and on the workbench about 10 minutes to punch out the broken pin, and separate the crankshaft parts.

Crankshaft, big ends, eccentric rods, main bearings and flywheel removed.
The crankshaft with undisturbed eccentrics, set up on 2 V blocks on a granite surface plate. With the 2 parts pushed together. But something was wrong. With the broken end clamped in the V block, the other end was held about a millimeter above the V block. WTF!

By this time the join had been cleaned with acetone, primed with Loctite 7471, and glued with Loctite Wicking 290. And reamed the hole to accept a number 1 taper pin.

So I checked the diameters of the mainshaft at both ends. 23.47mm at the broken end. 22.86 at the high end!!! Bugger. I should have checked before gluing. But why would the mainshaft have different end diameters???

Oh well! I decided, foolishly with hindsight, to reassemble the whole engine and see if the discrepancy was noticeable.

Next day, another 4 hours, and the reassembly was complete.

Rotated the flywheel. And it was horrible!! The flywheel runout was not “noticeable”. It was horrible!!

It had to be redone. Or do I just bite the bullet and make a new crankshaft?

I decided to redo the repair job, lining up the parts in the lathe.

Long story short again… teardown was much quicker this time. Experience counts.

This time I took a very light skim off the shaft and face using a very sharp cutter, to ensure that the ends and roughness were removed. Then held the broken shaft in a collet chuck which I know is very accurate. But found another problem. The shaft at the other end of the crankshaft had not only a smaller diameter, but was also at a slight angle axially, so I could not use the machined centre in the end of the shaft. So I set up the fixed lathe steady pictured, mounting it at the main bearing location. Trouble was that I had no accurate method of centering the steady. I described this setup to my engineering group, and was informed that I should have used a set up rod machined to the diameter of the end of the crankshaft, to set the position of the steady. Makes sense. My bad.

Oh well. I will reassemble the engine again. If it is again horrible, I will either do the whole job again, properly this time, OR MAKE A NEW CRANKSHAFT.

I have a feeling that I will be making a new crankshaft.

p.s. I allowed a day for the Loctite to cure, then deeply reamed the existing hole, and reinserted the taper pin in the enlarged tapered hole. This time the head was buried, but there should be enough purchase to remain intact.

Reassembled the engine, and turned it over to check the flywheel wobble.

I will not claim that it is perfect, but it is very close. I will not start making a new crankshaft just yet, but that is the next step if this repair is eventually unsatisfactory.

Boiler recertification next week.